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Fruit softening is a key component of the irreversible ripening program, contributing to the palatability necessary for frugivore-mediated seed dispersal. The underlying textural changes are complex and result from cell wall remodeling and changes in both cell adhesion and turgor. While a number of transcription factors (TFs) that regulate ripening have been identified, these affect most canonical ripening-related physiological processes. Here, we show that a tomato fruit ripening–specific LATERAL ORGAN BOUNDRIES ( LOB ) TF, SlLOB1 , up-regulates a suite of cell wall–associated genes during late maturation and ripening of locule and pericarp tissues. SlLOB1 repression in transgenic fruit impedes softening, while overexpression throughout the plant under the direction of the 35s promoter confers precocious induction of cell wall gene expression and premature softening. Transcript and protein levels of the wall-loosening protein EXPANSIN1 ( EXP1 ) are strongly suppressed in Sl LOB1 RNA interference lines, while EXP1 is induced in Sl LOB1 -overexpressing transgenic leaves and fruit. In contrast to the role of ethylene and previously characterized ripening TFs, which are comprehensive facilitators of ripening phenomena including softening, Sl LOB1 participates in a regulatory subcircuit predominant to cell wall dynamics and softening. 

In seed plants, 1‐aminocyclopropane‐1‐carboxylic acid (ACC) is the precursor of the plant hormone ethylene but also has ethylene‐independent signaling roles. Nonseed plants produce ACC but do not efficiently convert it to ethylene. InArabidopsis thaliana, ACC is transported by amino acid transporters, LYSINE HISTIDINE TRANSPORTER 1 (LHT1) and LHT2. In nonseed plants,LHThomologs have been uncharacterized.

Here, we isolated an ACC‐insensitive mutant (Mpain) that is defective in ACC uptake in the liverwortMarchantia polymorpha. Mpaincontained a frameshift mutation (1 bp deletion) in the MpLHT1coding sequence, and was complemented by expression of a wild‐type MpLHT1transgene. Additionally, ACC insensitivity was re‐created in CRISPR/Cas9‐Mplht1knockout mutants. We found that MpLHT1 can also transportl‐hydroxyproline andl‐histidine.

We examined the physiological functions of MpLHT1in vegetative growth and reproduction based on mutant phenotypes. Mpainand Mplht1plants were smaller and developed fewer gemmae cups compared to wild‐type plants. Mplht1mutants also had reduced fertility, and archegoniophores displayed early senescence.

These findings reveal that MpLHT1 serves as an ACC and amino acid transporter inM. polymorphaand has diverse physiological functions. We propose that MpLHT1 contributes to homeostasis of ACC and other amino acids inM. polymorphagrowth and reproduction.